Capacitively coupled conveyer measuring system

ABSTRACT

A conveyor and a sensing system for sensing various conditions on an advancing conveying bodies of a conveyor. The conveyor includes an array of sensing elements embedded in the conveying bodies to measure belt conditions. The sensing elements form parts of passive resonant circuits that each include a capacitor and an inductive coil. The capacitor or the inductive coil can be a sensing element. Measuring circuits external to the belt are inductively or capacitively coupled to the resonant circuits in the conveying bodies as they pass closely by. The sensing elements change the resonant frequency of their resonant circuits as a function of the sensed conditions. Frequency detectors in the measuring circuits measure that frequency change and convert it into a functionally related value used to determine a conveyor condition. Exemplary conditions include temperature, pressure, humidity, spillage, and product weight.

BACKGROUND

The invention relates generally to power-driven conveyors and moreparticularly to conveyor systems using sensing elements embedded in aconveyor body to detect conditions affecting the conveyor.

Sensors embedded in conveyor belts and other kinds of conveyors withmoving article-supporting conveying bodies require power to make sensormeasurements and to transmit measurements off the belt. On-beltbatteries, storage capacitors, and energy-harvesting devices have beenused or proposed for that purpose. But most of these solutions requirerecharging or replacement, take up space, add weight, or weaken thebelt. Rip detectors are used in flat belts to determine if a belt hasdeveloped a tear and is in danger of imminent failure. Many ripdetectors include thin wire loops embedded in a belt. When an untorn,closed wire loop passes a detector along the conveying path, a detectionsignal is generated and detected by the detector. When a torn, open wireloop passes, the detection signal differs from that for an untorn loop,indicating a rip in the belt. Although these devices do not requireon-belt power sources, they are not designed to make a continuum ofsensor measurements. Rather they generate a detection signal that isused for a binary conclusion: torn or not torn.

SUMMARY

One version of a conveyor belt embodying features of the inventioncomprises a plurality of resonant circuits disposed at sensor positionsin a belt body. Each of the resonant circuits includes an inductor and acapacitor connected to the inductor to form the resonant circuit. Theresonant frequency of each resonant circuit is determined by theinductance of the inductor and the capacitance of the capacitor. Atleast one of the inductance of the inductor and the capacitance of thecapacitor is varied by a varying condition affecting the conveyor belt.

In another aspect, one version of a conveyor-belt measuring systemcomprises a conveyor belt and at least one stationary measurementcircuit external to the conveyor belt. The conveyor belt includes aplurality of resonant circuits disposed at sensor positions in theconveyor belt. Each of the resonant circuits has a resonant frequencyand includes a sensing element sensing a condition affecting theconveyor belt and changing the resonant frequency as a function of thecondition affecting the conveyor belt and a coupling element connectedto the sensing element. The stationary measurement circuit includes afrequency detector and a stationary coupling element coacting with thecoupling elements in the conveyor belt as they pass close to thestationary coupling element to couple the resonant circuits to thefrequency detector. The frequency detector measures changes in theresonant frequency of the resonant circuits caused by the conditionaffecting the conveyor belt.

Another version of a conveyor-belt measuring system comprises a conveyorbelt and at least one stationary measurement circuit external to theconveyor belt. The conveyor belt includes a plurality of sensingelements disposed at sensor positions in the conveyor belt. Anelectrical property of each of the sensing elements is changed by acondition affecting the conveyor belt. The at least one stationarymeasurement circuit includes a frequency detector and a stationarycoupling element that couples the at least one stationary measurementcircuit to the plurality of sensing elements in the conveyor belt asthey pass closely by. Each of the sensing elements forms part of aresonant circuit having a resonant frequency that depends on theelectrical property of sensing element. The frequency detector measureschanges in the resonant frequency of the resonant circuit caused by thecondition affecting the conveyor belt.

Yet another version of a conveyor-belt measuring system comprises aconveyor belt and at least one stationary measurement circuit externalto the conveyor belt. The conveyor belt includes a plurality ofdeflection sensors disposed at sensor positions in the conveyor beltconnected to an associated coupler. Each of the deflection sensorssenses deflection of the conveyor belt. Each stationary measurementcircuit includes a stationary coupler coupling the stationarymeasurement circuit to the plurality of deflection sensors in theconveyor belt without contact as the associated couplers in the conveyorbelt pass close to the stationary coupler The stationary measurementcircuit measures the deflection of the conveyor belt sensed by each ofthe deflection sensors.

And another version of a conveyor-belt measuring system comprises aconveyor belt and at least one stationary measurement circuit externalto the conveyor belt. The conveyor belt includes a plurality of variablecapacitors disposed at sensor positions in the conveyor belt. Thecapacitance of each of the variable capacitors is changed by a conditionaffecting the conveyor belt. A first plate and a second plate areconnected in series with each of the variable capacitors. The stationaryexternal measurement circuit includes a first stationary plate forming afirst coupling capacitor with the first plate in the conveyor belt and agrounded second stationary plate forming a second coupling capacitorwith the second plate in the conveyor belt as the sensor position in theconveyor belt passes close to the stationary measurement circuit. Aresistor in the external measuring circuit is connected to the firststationary plate of the first coupling capacitor in series with anoscillator, the second coupling capacitor, and the variable capacitor.The oscillator produces an alternating voltage. A rectifier connected tothe junction of the resistor and the first coupling capacitor rectifiesthe alternating signal at the junction. A lowpass filter is connected tothe rectifier to produce a dc signal whose amplitude varies inverselywith the capacitance of the variable capacitor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a portion of a conveyor belt embodyingfeatures of the invention;

FIG. 2 is an isometric view of a conveyor system using a conveyor beltas in FIG. 1;

FIG. 3 is a top plan view of a portion of the conveyor system of FIG. 2partly cut away;

FIG. 4 is a cross section of the conveyor system of FIG. 3 taken alonglines 4-4 and showing a capacitive force-sensitive element;

FIG. 5 is a view of the belt portion of FIG. 4 shown with a downwardforce applied to deform the capacitor;

FIG. 6 is a block diagram of a distributed sensor usable in the conveyorsystem of FIG. 2;

FIG. 7 is an electrical schematic/block diagram of a sensor as in FIG.6;

FIG. 8 is a top schematic of the conveyor system of FIG. 2;

FIGS. 9A and 9B are cross sections of a belt portion in a conveyorsystem as in FIG. 3 showing another version of a capacitiveforce-sensitive element in the absence and in the presence of adownward-acting force on the belt;

FIGS. 10A and 10B are cross sections of a belt portion in a conveyorsystem as in FIG. 3 showing an inductive force-sensitive coil in theabsence and in the presence of a downward-acting force on the belt;

FIG. 11 is a cross section as in FIG. 10B in which the core of the coilchanges the inductance of the coil when a downward-acting force acts onthe belt;

FIG. 12 is an electrical schematic of an alternative inductively coupleddifferentiating-amplifier circuit usable in a sensor system as in FIG.6;

FIG. 13 is a cross section as in FIG. 4 showing an alternative sensorarrangement with capacitive coupling;

FIG. 14 is an electrical schematic of one version of sensor circuitryusable with an arrangement as in FIG. 13;

FIG. 15 is an electrical schematic of an alternative version of thesensor circuitry of FIG. 14;

FIG. 16 is a cross section as in FIG. 13 of another version of a sensorarrangement with capacitive coupling;

FIGS. 17A-17B, 18A-18B, 19A-19B, and 20A-20B are side views ofcapacitive temperature-sensing elements whose plate separation changeswith temperature;

FIGS. 21A-21B, 22A-22B, and 23A-23B are side views of capacitivetemperature-sensing elements whose dielectric changes with temperature;

FIGS. 24A-24B, 25A-25B, and 26A-26B are side views of inductivetemperature-sensing elements whose core changes with temperature;

FIGS. 27A-27B and 28A-28B are side views of inductivetemperature-sensing elements whose coil dimensions change withtemperature;

FIGS. 29A-29B and 30A-30B are side views of inductive pressure-sensingelements whose coil dimensions change with pressure;

FIGS. 31A-31B, 32A-32B, and 33A-33B are side views of capacitivepressure-sensing elements whose plate separation changes with pressure;

FIGS. 34A-34B and 35A-35B are side views of capacitive sensing elementswhose plate separation changes with humidity or spillage;

FIGS. 36A-36B, 37A-37B, 38A-38B are side views of inductive sensingelements whose coil or core dimensions change with humidity or spillage;

FIGS. 39A-39B are side views of a capacitive sensing circuit whosepermittivity is changed by spillage; and

FIG. 40 is a schematic diagram of another version of a measuring circuitusable in a conveyor system as in FIG. 2.

DETAILED DESCRIPTION

A portion of one version of a conveyor belt embodying features of theinvention is shown in FIG. 1. The conveyor belt 10 is a modular plasticconveyor belt constructed of a series of rows 12 of one or more plasticbelt modules 14 hingedly connected end to end by hinge rods 16 or pinsin interleaved hinge elements 17 forming hinge joints 18 betweenconsecutive rows. The belt modules 14 are conventionallyinjection-molded out of a thermoplastic polymer, such as polypropylene,polyethylene, acetal, or a composite polymer. Sensing elements, such asforce-sensitive elements 20, are embedded in the conveyor belt 10 atindividual positions. In this version, the force-sensitive elements arearranged in a two-dimensional array of rows R across the width of eachbelt row 12 and columns C along the length of the belt in a conveyingdirection 22. In this way the position of any individual force-sensitiveelement 20 can be defined as P_(RC), where R represents the row (or beltrow if each belt row has only one row of force-sensitive element) and Crepresents the column from one side of the belt to the other. Thedensity of the array or the separation between rows and columns offorce-sensitive elements for a given belt may be determined with apriori knowledge of the sizes and shapes of the conveyed articles. Inthis version each force-sensitive element 20 is mounted at the outerconveying surface 24 of the belt 10. The force-sensitive elements may beprotected by a cover 26 that may be domed to form a salient protrusionslightly above the belt's conveying surface 24 so that the entire weightof a conveyed article is borne by a group of the covers. Theforce-sensitive elements have sensing axes 28 that are perpendicular, ornormal, to the conveying surface 24 to measure forces applied normal tothe conveying surface at the positions of the force-sensitive elementson the belt.

The conveyor belt 10 is shown in a weighing system 30 in FIG. 2. Theconveyor belt advances in the conveying direction 22 along an uppercarryway 32. The endless belt is trained around drive and idle sprocketsets 34, 35 mounted on shafts 36, whose ends are supported in bearingblocks 38. A drive motor 40 coupled to the drive shaft rotates the drivesprockets 34, which engage the underside of the belt and drive the beltin the conveying direction 22 along the upper carryway 32. The beltreturns along a lower returnway 42. Rollers 44 support the belt in thereturnway and reduce the maximum catenary sag.

As shown in FIG. 3, the conveyor belt 10 is supported along the carrywayatop wearstrips 50. Activation circuits 52 for the force-sensitiveelements 20 are housed in housings 54 whose top surface is at orslightly below the level of the tops of the wearstrips 50. Theactivation circuits 52 are arranged in columns aligned with columns C ofthe force-sensitive elements 20 in the conveyor belt 10. Conveyingbodies 53 represent individual belt modules or belt rows in theconveyor-belt example used to describe the operation of the embeddedsensors, but the conveyor bodies shown in FIG. 3 also represent anyarticle-carrying conveyor modules advanceable in a conveying directionby a drive system. Examples of other article-carrying conveyor modulesinclude slats in a slat conveyor and trays in a tray conveyor and trays,or pucks, in an electromagnetic-rail conveyor, in which sensors could beembedded and used similarly.

A cross section of one belt row is shown in FIG. 4. Embedded in theplastic belt module 55 is a capacitor 56 having an upper plate 58 and alower plate 59. The two plates are shown parallel to each other and tothe top conveying surface 24 of the belt in the absence of articles atopthe belt. The plates are electrically wired to opposite ends of acoupler, or coupling element, an inductive coil 60 wrapped around abobbin 62 near the bottom side 25 of the belt. The capacitor iselectrically connected across the inductive coil 60 to form a passive,high-Q resonant circuit 61. The external activation circuit 52 includesan oscillator and an activation, or coupling, coil 64 that coacts withthe belt coil 60 passing by. In the vicinity of the closest point ofapproach, the two coils are positioned close enough across a gap for theactivation coil to couple inductively to the passive resonant circuit inthe belt wirelessly without physical contact. The activation coil 60 isconnected to support electronics 66 in the housing 54.

As shown in FIG. 5, a downward force F, such as produced by the weightof a conveyed article sitting atop the cover 26, causes the upper plate58 of the capacitor 56 to deflect or move downward. The reduction in theseparation S between the deflected upper plate 58 and the rigid, fixedlower plate 59 causes the capacitance to proportionally increase becausecapacitance is inversely proportional to the distance between theplates. And because the movement of the upper plate is proportional tothe applied force F, the capacitance is proportional to the forceapplied to the cover 26 by a supported article. Thus, the capacitor 56is a force-sensitive, or deflection-sensitive, element in the example ofFIG. 5. Any change in capacitance causes a change in the resonantfrequency of the passive L-C circuit formed by the inductive coil 60 andthe capacitor 56. Together, the resonant circuit 61 in the belt and theoscillator in the external circuit 66 form a distributed sensor—in thiscase, equivalent to a load cell or a deflection sensor—with asensing-circuit portion in the belt and a measuring-circuit portionexternal to the belt.

Another version of a force-sensitive capacitor is shown in FIGS. 9A and9B. In this version the plates 110, 111 of the capacitor 112, i.e., theplanes defined by the plates, are generally perpendicular to the topsurface 24 of the conveyor belt. The first plate 110 is verticallymovable, while the second plate 111 is rigidly fixed in place. When nodownward force acts on the protrusion 26, the two plates 110, 111 arevertically offset from one another as shown in FIG. 9A. When a downwardforce F is applied by the weight of a conveyed article, as in FIG. 9B,the movable plate 110 moves downward 114 increasing the area between thetwo plates and proportionally increasing the capacitance and reducingthe resonant frequency of the L-C circuit. Of course, the two platescould alternatively be positioned in parallel with no vertical offset inthe absence of a force. In such a design, a downward force would pushone plate downward relative to the other to increase the offset,decrease the area between the plates, decrease the capacitance, andincrease the resonant frequency of the L-C circuit. Like the capacitor56 in FIG. 4, the capacitor 112 of FIGS. 9A and 9B is connected acrossthe inductive coil 60.

FIGS. 10A and 10B show another version of a force-sensitive resonantcircuit. In this version, an inductive coil is the force-sensitiveelement. The coil 120 is electrically connected to a fixed-capacitancecapacitor 122 to form a passive resonant L-C circuit. When a downwardforce F is applied to the protrusion 26, the coil 120′ is compressedlike a spring, as shown in FIG. 10B. The decreased length of the coilincreases the inductance of the coil 120′ and reduces the resonantfrequency of the L-C circuit. A downward force increasing the crosssection of the coil would also increase the inductance of the coil 120.In fact, other force-induced changes in the geometric shape of the coilcan affect its inductance and the resonant frequency of the passive L-Ccircuit.

FIG. 11 shows an alternative force-sensitive coil 124. The coil'sgeometry is fixed, but a downward force F on the protrusion 26 pushes ametallic core 126 with a high permeability downward deeper into the coil124. The increased penetration depth of the core increases thepermeability and the inductance of the coil 124 and decreases theresonant frequency of the L-C circuit formed by the coil and a fixedcapacitor 130. Alternatively, the inductance can be increased by movinga conductive ring closer to or encircling more windings of the coil orby moving a conductive plate closer to the coil, or both. Of course, thecore, plate, or ring could be arranged relative to the coil to decreasethe inductance and increase the resonant frequency under the influenceof a force on the conveyor belt that decreases the penetration of thecore or increases the distance of the plate or ring from the coil.

A block diagram of one sensor, or load cell, of a weighing system isshown in FIG. 6. The load cell includes the passive resonant circuit 61in the belt forming the sensing circuit and an oscillator 68, includingthe activation coil, in the external housing. The sensing circuit in theconveyor belt may be made of discrete electrical components embedded inthe belt, or it may be made smaller using microelectromechanical-systems(MEMS) technology. The oscillator is set to oscillate at a frequencythat is close to the resonant frequency of the resonant circuit 61 inthe belt. When the resonant circuit is near the activation coil, thecoil, acting as an antenna, inductively couples the resonant circuit 61to the oscillator 68. The interaction of the resonant circuit 61 withthe oscillator 68 changes the oscillator frequency in accordance withthe capacitance change in the resonant circuit. The frequency of theoscillator 68 is measured by a frequency detector 70. The oscillator andthe frequency detector 70, which form the measuring circuit of thedistributed load cell, are powered by a power supply 72. The change infrequency of the oscillator is proportional to the downward force on thecapacitor 56. The frequency detector's output is converted into a weightand recorded locally or remotely in a data recorder 74.

A more detailed circuit diagram is shown in FIG. 7. The passive resonantcircuit 61 includes the capacitor 56, whose capacitance changes with anapplied force, and the coil 60. The coil 60 is an inductor withinductance L₁, and the capacitor 56 has a capacitance C₁ that is afunction of the applied force. The coil also has a small seriesresistance. No power source is needed in the belt. The resonantfrequency (in Hertz) is given by f_(r)=1/[2π(L₁C₁)^(1/2)]. The externaloscillator 68 includes the activation coil 64 with inductance L₂ and asmall resistance not depicted in FIG. 7. A capacitor 75 having a fixedcapacitance in parallel with a trim capacitor 77 having a manuallyvariable capacitance is connected between one end of the activating coil64 and ground. The combined capacitance of the fixed and variablecapacitors 75, 77 is C₂. The other end of the coil 64 is connected tothe non-inverting input (+) of an operational amplifier (op amp) 94. Thejunction of the coil 64 and the capacitors 76, 78 is connected to theinverting input (−) of the op amp 94. Positive feedback is applied byconnecting the output 96 of the op amp 94 to its non-inverting input (+)to maintain oscillation at the nominal oscillator frequency given byf_(n)=1/[2π(L₂C₂)^(1/2)]. The trim capacitor 77 is adjusted to set thenominal oscillator frequency, i.e., the frequency of the oscillator whenuncoupled from the resonant circuit 61 in the belt, to a value close tothe resonant frequency f_(r) of the resonant circuit 61.

The op amp 94 is operated single-ended with its upper voltage rail at apositive voltage V (e.g., +5 Vdc) and its lower voltage rail at ground.The positively biased sinusoidal waveform 98 produced by the oscillator68 is buffered in an emitter-follower op amp 100 circuit serving as abuffer amplifier with high input impedance so as not to load theoscillator circuit. The buffered oscillator signal is applied to thefrequency detector. The frequency detector may be realized with analogand digital logic circuits or with a microcontroller.

In the example shown in FIG. 7, the frequency detector is realized witha microcontroller 71. The buffered oscillator waveform is applied to thenegative input AIN1 of the microcontroller's analog comparator. Thepositive input AIN0 is connected to the wiper arm of a potentimeter 102forming an adjustable voltage divider with the supply voltage V.Whenever the amplitude of the oscillator waveform at the negativecomparator input AIN1 crosses the threshold voltage at the positivecomparator input AIN0 set by the potentiometer, an interrupt isgenerated in the microcontroller. The interrupt is serviced by afirmware routine that increments a counter counting the number of cyclesof the oscillator waveform. The total cycle count in a predeterminedtime interval is proportional to the oscillator frequency. The cyclecount is reset to zero at the start of the next interval. Thus, thefrequency detector is realized as a frequency counter in this example.But other methods of detecting the frequency could be used. For example,the microcontroller could be a digital-signal-processing (DSP) devicecapable of performing Fast Fourier Transform (FFT) or Fast HartleyTransform (FHT) algorithms on the oscillator waveform to extract itsfrequency. In that case, the frequency detector is realized as aspectrum analyzer.

When the resonant circuit 61 in the belt is far from the oscillator 68,the oscillator's nominal frequency f_(n) is unaffected by the resonantcircuit. As the belt advances and the resonant circuit 61 comes in closeproximity to the oscillator 68, the interaction between the two circuitsincreases. The oscillator's frequency changes from its nominal frequencyf_(n). The frequency detector detects that change in frequency. When thefrequency detector is implemented as a frequency counter in amicrocontroller as previously described, the cycle count in thepredetermined interval is a measure of the force acting on the capacitor56 in the belt. Because the frequency change is also a function of theproximity of the belt coil 60 to the oscillator coil 64, amicrocontroller routine reports the maximum change in frequency fromnominal as the best measure of the force applied to the belt capacitor56. The microcontroller converts the cycle count to a weight value. Themicrocontroller 71 may be connected to a user interface including anoutput display 104 and a manual input device, such as a keyboard 106.The microcontroller 71, along with the microcontrollers in the otheractivation units, is also connected directly or wirelessly to a maincontroller 108.

A vision system as in FIG. 1 includes a camera 76 or other opticaldetector supported above the carryway 32 to vision a portion of theconveying surface. Optical signals 107 from the camera are sent to amain controller 108. The main controller executes a pattern-recognitionprocess to determine the footprints of individual articles conveyed onthe belt from the optical signals. With a priori knowledge of theload-cell-array geometry relative to a point on the belt, such as abelt-module corner 78, the vision system can determine relatively agroup of load cells under an individual article's footprint. Forexample, in the portion of the conveying surface 24 as shown in FIG. 8,article B overlies ten load cells covering two columns C and five rowsR. Optically detectable markers 80 on some or all belt rows, forexample, may be used by the vision system to identify absolutely whichten sensing elements are covered by article B. In this example, thevision system reads the marker, which may be coded or may simply be thenumber 10 indicating that it is on row 10 of the belt. With the a prioriknowledge of the array geometry and the footprint of article B withrespect to row 10, the vision system can identify the ten sensingelements underlying the article. The vision system can then execute aweighing process that combines the measurements of the force-sensingelements at absolute positions P_(RC)={(R, C)}={(11, 2); (11, 3); (12,2); (12, 3); (13, 2); (13, 3); (14, 2); (14, 3); (15, 2); (15, 3)} tocompute the weight of article B. The load-cell measurements may becombined, for example, by summing the individual load-cell measurementsto compute a value equal or proportional to the weight of the underlyingarticle. Each of the articles is marked with identifying indicia 82,such as a bar code, that a reader in the vision system can interpret. Inthat way, the computed weight can be associated with a specificindividual article. And, because the vision system visions the entirewidth of the belt, articles do not have to be arranged in a single fileover a static weighing station in the carryway. Furthermore, theresonant-circuit array in the belt allows the weight to be measuredwithout stopping the belt. A video display of the vision system may beused to monitor system operating conditions and settings or the weightsof individual articles. The controller 108 may be a programmable logiccontroller, a laptop, a desktop, or any appropriate computer devicecapable of executing the processes described.

The vision system could use other means to assign weights to individualarticles. For example, the positions of each of the sensing elements, orload cells, could be marked on the conveying surface 24 or the load-cellcovers 26. The mark could identify each load cell explicitly or couldjust be a generic position mark, such as a dot 88 (FIG. 1) on each or apredetermined subset of the sensing circuits. If all the sensing-elementpositions are marked, the vision system would not need a prioriknowledge of the array layout. As another example the vision systemcould alternatively select all those sensing elements in an enlargedregion 90 (FIG. 8) about the article footprint and sum theirmeasurements. The load cells not under the article D would yieldmeasurement values of zero, which would not add to the weight. Thisensures that the entire article is weighed accurately. If, of course,another article is close by, the enlarged region has to be carefullyselected so as not to encompass the nearby article.

If the articles are separated enough so that no two articles are atopthe same or adjacent load cells, the weight of each article can bedetermined by summing the load-cell measurements of contiguous,non-zero-reading load cells.

The load-cell sensors can also be used to determine the center ofgravity (COG) of a conveyed article from the force measurements of eachof the contiguous, non-zero-reading load cells and a priori knowledge ofeach load cell's relative position via conventional COG formulas. And,more simply, the load cells can be used as position sensors to determinethe positions of articles on the belt for improved tracking of divertedarticles or to detect article skew on the belt from the pattern ofcontiguous non-zero-reading load cells under a conveyed article.

To minimize variations in the outputs of the sensors and improve theiraccuracy, the sensors can be calibrated. The calibration can be manualor automatic. In manual calibration, the responses of each of thesensors to known conditions can be used to determine calibration values(e.g., gains and offsets) that can be stored in memory accessible by themain controller or by each sensor's microcontroller. The calibrationvalues are used to correct the measurements. In automatic calibrationthe sensors can be calibrated periodically while the conveyor belt isrunning. Each of the sensors is subjected to known conditions. Thecalibration can start at a known point in the belt by using a known beltlength to ensure that all the sensors are calibrated. A reference at aknown position in the belt relative to the sensors can be sensed andused to start and stop calibration. By measuring the responses of thesensors over several belt circuits, a signature map of the sensors canbe developed. The signature map is used to assign the calibration valuesto the corresponding sensors.

FIG. 12 shows an alternative inductively coupled resonant circuit formeasuring the change in the resonant frequency of an L-C circuit causedby a change in the capacitance of a sensing capacitor 130. The sensingcapacitor 130 and an inductive coupling coil 132 in the belt form aresonant L-C circuit 133. The external stationary activation andmeasuring electronic circuit in the conveyor frame has a coil 134inductively coupled to the coil 132 of the L-C circuit. A first op amp136 is configured as a differentiating amplifier with capacitors 138,139 between the inductive coil 134 and the op amp's inverting (−) andnon-inverting (+) inputs and a feedback resistor 140. A second op amp142 with its resistors 144, 145 forms an inverting amplifier, whoseoutput is directed back to the coil 134 by activation line 146 toactivate the L-C circuit 133 in the belt. The output 148 of the secondop amp 142 is connected to a frequency detector, such as the frequencycounter 71 shown in FIG. 7, after any signal conditioning required tolimit the maximum and minimum voltage levels of the output signal to arange appropriate for the frequency counter's input. Unlike themeasuring system of FIG. 7, this measuring system measures the resonantfrequency of the L-C circuit directly rather than the frequency of aseparate oscillator whose frequency is changed from its nominal resonantfrequency by interaction with a passive-LC circuit passing closely by.

A capacitively coupled resonant circuit is shown in FIG. 13. Acapacitive sensor 150 is mounted in a conveyor belt 152. The capacitanceof the sensor 150 changes as a function of some condition experienced bythe belt. The plates 154, 155 of the capacitive sensing element 150 areconnected to opposite terminals of an inductive coil 156 to form aresonant L-C circuit. The plates 154, 155 are also connected to couplingelements, in the form of plates 158, 159, near the bottom of the belt152. The belt's coupling plates 158, 159 form coupling capacitors 160,161 with corresponding stationary plates 162, 163 mounted in theconveyor frame 164 external to the belt 152. The stationary plates 162,163 are connected to a stationary resonator-activation-and-measuringcircuit 166, as also shown schematically in FIG. 14. A first op amp 168is configured as a differentiating amplifier by an input capacitor 170and a negative-feedback resistor 172. A second op amp 174 is configuredas an inverting amplifier with resistors 176, 177. The inverter's output178 is connected to a frequency detector (not shown). The inverterinverts the signal out of the differentiating amplifier so that itsoutput 178 is in-phase with the oscillations of the L-C circuit. Theoutput 178 is fed back to the L-C circuit over a feedback path 180 viastationary plate 163. A grounded resistor 182 connected to the otherstationary plate 162 allows for draining and charging of thedifferentiating capacitor 170 so that a measurable signal is presentedto the op amp 168. To reduce the effect of stray capacitance on themeasuring circuit, the capacitance of the coupling capacitors 160, 161should be much greater than the stray capacitance. But if that resultsin physically large and impractical capacitors, a compensation capacitor184 may be inserted in the feedback path 180 to minimize measurementnoise caused by changes in the capacitance of the coupling capacitors160, 161. The compensation capacitor 184 preferably has a smallcapacitance and is located close to the plate 163 to reduce the effectof stray capacitance on the measurement.

Another capacitively coupled circuit 166′ is shown in FIG. 15. In thiscircuit a first op amp 186 is configured as a differential comparatorwith the input coupling capacitors 160, 161 and feedback resistors 188,189. A second op amp 190 is configured as an inverting amplifier withits resistors 192, 193 setting the gain. The inverter aligns the phaseof the output 194 with the L-C oscillation. The output is fed back tothe comparator through the feedback resistor 188 and is sent to thefrequency detector (not shown). A resistor 196 connected between theinverting (−) and non-inverting (+) inputs of the first op amp 186prevents the inputs from floating.

FIG. 16 shows a capacitively coupled sensor arrangement in which aconveyor belt 200 has a metal plate 202 at the belt's bottom 204. Theplate 202 is spring-loaded, in this example, between leaf springs 203,203′ connected by a beam 205, such that a weight applied to the platedeflects the springs and moves the plate vertically downward. Stationaryplates 206, 207 in the conveyor frame 208 form two variable-capacitancehalf-capacitors with the metal plate 202. The plates 206, 207 areconnected to an inductor in an activation-measurement circuit 210 in theconveyor frame. The two capacitors connected in series and the inductorform an L-C resonator. The resonant frequency changes with the verticalposition of the elongated plate 202, which affects the capacitors' plateseparation. Because, for practical sizes of coupling capacitors, thecoupling range of capacitors is less than that of practical-sizedcoupling inductors, capacitive coupling can provide a more preciseestimate of the position of a passing sensing element. So, in thisexample, the sensing element formed by the spring-loaded plate 202 alsoserved as the coupling element coupling the plate 202 to the externalmeasurement circuit 210 via the stationary plates 206, 207. And the L-Cresonant circuit is distributed between the sensing element 202 in theconveyor belt 200 and the external stationary measurement circuit 210.

In all the examples already described, the resonant frequency of an L-Ccircuit was changed by a force acting on a capacitor or an inductor. Inparticular, the examples were described as force-sensitive ordeflection-sensitive elements whose electrical property, capacitance orinductance, is changed by the deflection of a conveyor belt caused bythe weight of a conveyed article atop the belt. But it is possible touse similar resonant circuits to measure quantities other than productweight. In the examples that follow, variable capacitors used as sensingelements are connected to fixed-value inductors (not shown) to provideresonant L-C circuits, and variable inductors used as sensing elementsare connected to fixed-value capacitors (not shown) to provide resonantL-C circuits.

FIGS. 17A-23B show a variety of variable capacitors used as sensingelements in a conveyor belt to sense temperature. FIG. 17A shows acapacitive sensor constructed of two parallel plates 212, 213 spacedapart across a gap 214 by spacers 216 made of a material with a highcoefficient of thermal expansion. When the temperature rises, as in FIG.17B, the spacers 216 expand and increase the gap 214′. The capacitanceis reduced, which changes the resonant frequency of the L-C circuit inwhich the plates 212, 213 are connected.

Another capacitive temperature sensor is shown in FIGS. 18A and 18B. Thesensor comprises two plates 218, 219 separated by spacers 220 across agap 222 filled with a gas. The top plate 218 is flexible so that, as thetemperature rises or falls, the top plate can expand outward or sinkinward as the gas pressure increases or decreases. The outward expansionof the flexible top plate 218 with a rise in temperature is shown inFIG. 18B. The increased gap 222′ causes the capacitance to decrease,which increases the resonant frequency.

FIG. 19A shows another temperature-sensitive capacitive sensorconstructed of two plates 224, 225 separated by a gap 226. The upperplate 224 is connected at the edge to one end of a bimetallic strip 228pinned at the opposite end to structure in the belt. When thetemperature increases, as in FIG. 19B, the bimetallic strip 228 bends,moving the upper plate 224 away from the lower plate 225 and increasingthe gap 226′. The larger gap decreases the capacitance, which increasesthe resonant frequency. The sensor in FIG. 20A works in a similar way,except that one edge of the upper plate 224 is connected to one end ofan arm 228 attached to a pivot 230 at the other end. A pusher 232contacts the arm 228 at a position along the arm's length between thepivot 230 and the plate 224. The pusher 232 is made of a material with ahigh coefficient of thermal expansion. When the temperature rises, as inFIG. 20B, the pusher elongates and tilts the arm 230 and the upper plate224 so that the gap between the plates 224, 225 increases, lowering thecapacitance.

FIGS. 21A-23B show other versions of capacitive sensors in which thecapacitance is changed by changes in the capacitors' effectivedielectric constants. In FIG. 21A a dielectric slab 234, affixed at oneend to structure in a conveyor belt, extends partway into a gap 236between upper and lower fixed plates 238, 239. The permittivity of theslab 234 differs from that of air. The slab 234 is also made of amaterial with a high coefficient of thermal expansion. When thetemperature rises, as in FIG. 21B, the slab 234 extends farther into thegap 236. The region in the gap 236 occupied by the dielectric slab 234increases with temperature. That causes the capacitance to increase andthe resonant frequency to decrease as the temperature increases. Asimilar effect is achieved in the versions shown in FIGS. 22A-23B.Instead of a dielectric material with a high coefficient of thermalexpansion, a fixed-size dielectric slab 240 is attached to one end of abimetallic strip 242. The other end of the strip 242 is affixed to beltstructure. When the temperature increases, the bimetallic strip 242bends, as shown in FIG. 22B, which pushes the dielectric slab 240 deeperinto the gap 236. The capacitive sensor in FIGS. 23A-23B uses a similarfixed-size dielective slab 240. Instead of a bimetallic strip, it uses apusher 244 and a pivot 230 and an arm 228 as in FIG. 20A to push theslab 240 farther into the gap 236 so that the dielectric material fillsmore of the gap to increase the effective permeability.

FIGS. 24A-26B show inductive temperature sensors whose inductanceschange because of temperature-induced changes in the effectivepermeability of inductors in L-C circuits due to the number of windingsencircling the cores. FIG. 24A shows an inductor coil 246 with a ferritecore 248 or a core made of another material extending through the centerof the coil a first distance. The core 248 is made of a material alsohaving a high coefficient of thermal expansion. When the temperaturerises, as in FIG. 24B, the core 248 expands to extend farther into thecoil 246 and link more of the coil's turns. The effective permeabilityand the inductance increase, and the resonant frequency of an L-Ccircuit including the coil 246 and a fixed capacitor decreases. Theversions in FIGS. 25A-25B use a bimetallic strip 250 to push afixed-length core 252 farther into the coil 246, as shown in FIG. 25B.In FIGS. 26A-26B the fixed-length core 252 is pushed by a pivotable arm228 urged by a pusher 244 made of a material with a high coefficient ofthermal expansion farther into the inductive coil 246 to change itsinductance as a function of temperature.

FIGS. 27A-28B show inductive temperature sensors that use materialshaving high coefficients of thermal expansion to change the shapes ofinductive coils. In FIG. 27A, an inductive coil 254 is wound around acore 256. The core is made of a material having a high coefficient ofthermal expansion so that, when the temperature increases as in FIG.27B, the core 256 expands radially and increases the diameter of thecoil 254, which increases the coil's inductance and lowers the resonantfrequency. In FIGS. 28A-28B an inductive coil 258 is attached at twopoints by arms 260, 261 to a slab 262 made of a material having a highcoefficient of thermal expansion. As shown in FIG. 28B, the slab 262elongates as the temperature increases. The elongating slab 262 drawsthe arms 260, 261 apart and lengthens the coil 258, which decreases thecoil's inductance and increases the resonant frequency.

Inductive pressure sensors shown in FIGS. 29A-30B operate analogously tothe temperature sensors of FIGS. 27A-29B. In FIGS. 29A-29B a coil 264 iswound around a closed, flexible pressure chamber 266 filled with a gas.When the external ambient pressure falls, as in FIG. 29B, the flexiblechamber expands radially and increases the diameter and cross-sectionalarea of the coil 264, which increases the coil's inductance. In FIG. 30Athe slab 262 of FIG. 28A is replaced by a gas-filled, expandablepressure chamber 268. The expansion of the chamber 268 with a fallingexternal pressure elongates a coil 270 as in FIG. 30B and decreases itsinductance.

Various capacitive pressure sensors are shown in FIGS. 31A-33B. A fixedlower plate 272 is separated from a flexible upper plate 273 by a wall274 forming a sealed chamber 276 with the plates. As shown in FIG. 31B,when the external pressure decreases, gas in the chamber 276 pushes theflexible upper plate 273 outward away from the fixed lower plate 272,which lowers the capacitance and increases the resonant frequency. InFIGS. 32A-32B the expansion of a gas in a closed chamber 278 pivots anarm 280 about a pivot 282 to separate plates 284, 285 of a capacitor asthe external pressure decreases. The separation of the plates decreasesthe capacitance. FIGS. 33A-33B depict another version using twoexpandable pressure chambers 286, 287 attached to a linkage 288. Thelinkage 288 is attached to an upper plate 290 of a capacitive sensor.Dips in the external pressure, as in FIG. 33B, cause the pressurechambers 286, 287 to expand and pivot the linkage 288 to separate thetop plate 290 from a fixed lower plate 291. The increased plateseparation decreases the capacitance.

The sensors shown in FIGS. 34A-37B sense changes in ambient humidity.The capacitive sensor of FIGS. 34A-34B comprises two parallel plates292, 293 separated by a separator 294 made of a hygroscopic material.The hygroscopic separator 294 expands with increasing humidity as shownin FIG. 34B. The expansion increases the separation between the plates,which decreases the capacitance and increases the resonant frequency.The sensor may also be used to detect spillage of wet materials. Thecapacitive humidity sensor of FIGS. 35A-35B operates like the capacitivetemperature sensor of FIGS. 20A-20B, except that the pusher 296 is madeof a hygroscopic material rather than a material with a high coefficientof thermal expansion. Increases in humidity cause the pusher 296 toelongate and separate the plates 298, 299 to decrease the capacitance.The inductive humidity sensors shown in FIGS. 36A-37B operate in thesame way as the temperature sensors of FIGS. 27A-28B except that thematerials made of high coefficients of thermal expansion are replaced byhygroscopic materials that expand with increased humidity. The expansionchanges the inductance of the coil by changing its length (FIG. 37B) orits diameter (FIG. 36B).

FIGS. 38A-38B show an inductive sensor that detects changes in humidityor spillage. A coil 300 has a core 302 made of a hygroscopic material.An increase in humidity in the presence of spillage causes the core 302to elongate as in FIG. 38B, which increases the coil's inductance.

FIGS. 39A-39B show a capacitive spillage sensor comprising two plates304, 305 having a dielectric slab 306 in a portion of a gap 308 betweenthe plates. The remainder of the gap 308 is filled with air in FIG. 39A.When a liquid leaks into the sensor, the liquid 310 displaces the air ina portion of the gap 308. Because the dielectric constant of thespillage differs from that of air, the effective permittivity of thecapacitor formed by the two plates changes. The change in permittivitywith spillage changes the capacitance and the resonant frequency. Thetwo plates 304, 305 are shown oblique to each other, diverging fromfirst ends of the plates to second ends, with the dielectric slab 306 inthe narrowest portion of the gap 308 between the first ends of theplates. Spilled liquid tends to collect against the dielectric slab 306by capillary action. In that way the spillage has a greater effectbecause it collects where the gap spacing is also smaller. But theplates can be parallel and still provide effective detection ofspillage.

In all these examples of sensing elements, except the one represented byFIGS. 39A-39B, a force changes the orientation, dimensions (i.e., sizeor shape), permeability, or permittivity of a passive component—aninductor or a capacitor—in a resonant L-C circuit. Any of these changesalso changes the inductance or capacitance and thus the resonantfrequency of the resonant circuit. So all those sensing elements may beconsidered to be force-sensitive elements. The change in the resonantfrequency is functionally related to the change in the force. And theforce is caused by the weight of an article on the belt, a change intemperature, a change in ambient pressure, a change in humidity, orspillage onto the belt. But those of ordinary skill in the art canappreciate that similar techniques to change the inductance orcapacitance of a circuit element can be used to detect other changingconditions affecting a conveyor belt.

Another version of a measuring system for measuring conditions in aconveyor belt that cause the capacitance of a capacitor to change isshown in FIG. 40. A variable capacitor 400 is embedded in a conveyorbelt 402. The capacitance of the variable capacitor 400 varies with acondition affecting the belt. The variable capacitor is connected inseries with a first plate 404 and a second plate 405. The two plates404, 405 are positioned close to the bottom of the conveyor belt 402.When the belt 402 passes close by a stationary external measuringcircuit 406, the first and second plates 404, 405 form first and secondcoupling capacitors 408, 409 with first and second stationary plates410, 411 in the external measuring circuit 406. The second stationaryplate 411 of the second coupling capacitor 409 is grounded. A resistor412 is connected in series with an oscillator 414, the two couplingcapacitors 408, 409, and the variable capacitor 400. The oscillator 414supplies an ac drive waveform at a fundamental frequency to adistributed series RC circuit composed of the resistor 412 and the threecapacitors 400, 408, 409. The ac drive waveform can be a sinusoidalwave, a triangular wave, or a square wave, for example. The amplitude ofthe ac voltage V at the junction of the resistor 412 and the firststationary plate is inversely proportional to the capacitance of thecircuit. Because the capacitance of the variable capacitor 400 is muchless than the capacitance of the two coupling capacitors 408, 409, theamplitude of the ac voltage V is inversely proportional to thecapacitance of the variable capacitor. The ac voltage V is rectified ina rectifier 416, such as a single diode forming a half-wave rectifier ora diode bridge forming a full-wave rectifier. The dc component of therectified voltage is proportional to the magnitude of the ac voltage Vat the resistor-coupling capacitor junction, or node, in the RC circuit.A lowpass filter 418 removes the oscillator fundamental frequency andharmonics and any high-frequency ripple caused by the rectifier toproduce a dc voltage at its output that is inversely proportional to thecapacitance of the variable capacitor 400. The analog dc voltage isconverted to digital values in an analog-to-digital converter 420. Thedigital values 422 are sent to a main controller (108, FIG. 7). Thelowpass filter 418 may be realized with only passive components or maybe realized as an active filter with passive components and operationalamplifiers. Buffer amplifiers may be inserted before and after thelowpass filter 418 to minimize circuit loading and reduce noise. So thismeasuring system differs from the others in that it measures a voltageamplitude rather than a frequency and does not use a resonant circuit.The fundamental drive frequency of the oscillator can be set over a widerange of frequencies (from kilohertz up to gigahertz) depending oncapacitance values and belt speed. The cutoff frequency of the lowpassfilter is set to about 0.1% to 1% of the fundamental drive frequency forgood results.

Although the weighing system has been described in detail with referenceto a few versions, other versions are possible. For example, theconveyor belt need not be a modular plastic conveyor belt. It could be aflat belt or a slat conveyor, for instance. As another example,visioning algorithms and detectable markers on the belt other than thosedescribed could be used by the vision system to identify individualarticles and the load cells underlying them. Furthermore, some of theactive components that make up the activation and measurement circuitsdescribed as external to the belt could instead be embedded in the beltwith couplers coupling the output from the belt. In such an arrangement,the active components would be powered by a belt-embedded power source,such as a battery, a storage capacitor, or an energy harvester, orthrough inductive coupling from a source external to the belt. Andtechniques other than resonant techniques can be used to determinechanges in inductance or capacitance due to belt deflections. So, asthese few examples suggest, the scope of the claims is not meant to belimited to the details of the exemplary versions.

What is claimed is:
 1. A conveyor measuring system comprising: aconveyor including: a plurality of article-carrying conveying bodiesadvanceable in a conveying direction; a plurality of deflection sensorsdisposed at sensor positions in the conveying bodies, wherein each ofthe deflection sensors senses deflection of the conveying body; and aplurality of couplers, each connected to an associated one of thedeflection sensors; wherein each of the couplers includes a first plateand a second plate both connected in series with the associateddeflection sensor; at least one stationary measurement circuit externalto the conveying bodies and including: a stationary coupler coupling theat least one stationary measurement circuit to the plurality ofdeflection sensors in the conveying bodies without contact as theassociated couplers in the conveying bodies pass close to the stationarycoupler; wherein the stationary coupler includes a first plate forming afirst coupling capacitor with the first plates of the couplers and asecond plate forming a second coupling capacitor with the second platesof the couplers as the couplers pass close to the stationary coupler;wherein the at least one stationary measurement circuit measures thedeflection of the conveying bodies sensed by each of the deflectionsensors.
 2. A conveyor measuring system comprising: a conveyorincluding: a plurality of article-carrying conveying bodies advanceablein a conveying direction; a plurality of deflection sensors disposed atsensor positions in the conveying bodies, wherein each of the deflectionsensors senses deflection of the conveying body; and a plurality ofcouplers, each connected to an associated one of the deflection sensors;at least one stationary measurement circuit external to the conveyingbodies and including: a stationary coupler coupling the at least onestationary measurement circuit to the plurality of deflection sensors inthe conveying bodies without contact as the associated couplers in theconveying bodies pass close to the stationary coupler; wherein the atleast one stationary measurement circuit measures the deflection of theconveying bodies sensed by each of the deflection sensors; wherein eachof the deflection sensors comprises a variable capacitor whosecapacitance varies with deflection of the conveying body; wherein thecoupler associated with the variable capacitor includes a first plateand a second plate both connected in series with the variable capacitor;wherein the stationary coupler includes a first plate forming a firstcoupling capacitor with the first plate of the coupler associated withthe variable capacitor and a grounded second plate forming a secondcoupling capacitor with the second plate of the coupler associated withthe variable capacitor; wherein the at least one stationary measuringcircuit includes: an oscillator producing an ac drive waveform; aresistor connected to the first plate of the first coupling capacitor inseries with the oscillator, the second coupling capacitor, and thevariable capacitor; a rectifier connected to the junction of theresistor and the first coupling capacitor to rectify the ac voltage atthe junction; a lowpass filter connected to the rectifier to produce adc voltage whose amplitude varies inversely with the capacitance of thevariable capacitor.
 3. A conveyor measuring system as in claim 2 whereinthe oscillator produces an ac drive waveform that is a sinusoidal wave,a triangular wave, or a square wave.
 4. A conveyor measuring systemcomprising: a conveyor including: a plurality of article-carryingconveying bodies advanceable in a conveying direction; a plurality ofvariable capacitors disposed at sensor positions in the conveyingbodies, wherein the capacitance of each of the variable capacitors ischanged by a condition affecting the conveying body; and a first plateand a second plate connected in series with each of the variablecapacitors; at least one stationary measurement circuit external to theconveying bodies and including: a first stationary plate forming a firstcoupling capacitor with the first plate in the conveying bodies and agrounded second stationary plate forming a second coupling capacitorwith the second plate in the conveying bodies as the sensor position inthe conveying bodies passes close to the stationary measurement circuit;an oscillator producing an ac drive waveform; a resistor connected tothe first stationary plate of the first coupling capacitor in serieswith the oscillator, the second coupling capacitor, and the variablecapacitor; a rectifier connected to the junction of the resistor and thefirst coupling capacitor to rectify the ac voltage at the junction; alowpass filter connected to the rectifier to produce a dc voltage whoseamplitude varies inversely with the capacitance of the variablecapacitor.